1. Field of the Invention
The present invention relates to an array of light-emitting elements having a multilevel wiring structure.
2. Description of the Related Art
The term xe2x80x98array of light-emitting elementsxe2x80x99 refers herein to a device in which a plurality of light-emitting elements are aligned in a row. If the light-emitting elements are light-emitting diodes (LEDs), the device is termed an LED array. LED arrays are used as exposure light sources in, for example, electrophotographic printers.
LED arrays are disclosed in, for example, the book LED Purinta no Sekkei (xe2x80x98Design of LED printers,xe2x80x99 Triceps). FIGS. 17 and 18, taken therefrom, show a sectional view and a top plan view of a conventional LED array.
In the LED array shown in FIGS. 17 and 18, an n-type gallium arsenide phosphide (GaAs0.6P0.4) layer 2 is formed on an n-type gallium arsenide (GaAs) substrate 1, and a p-type impurity such as zinc (Zn) is selectively diffused to form an array of light-emitting regions 3. Each light-emitting region 3 has an individual aluminum (Al) electrode 4, and all of the light-emitting regions 3 share a common gold-germanium-nickel (Auxe2x80x94Gexe2x80x94Ni) electrode 5. The gold-germanium-nickel electrode 5 is formed on and electrically connected to the underside of the n-type GaAs substrate 1. The aluminum electrodes 4 are formed on a dielectric layer 6 and are electrically connected to respective light-emitting regions 3. Each aluminum electrode 4 is also electrically connected to a wire-bonding pad, referred to below as an electrode pad 7.
FIG. 19 shows one cell in a conventional type of LED array with multilayer wiring. FIG. 20 shows an enlarged view of the part A in FIG. 19. The cell structure shown in 19 was recently proposed by the present inventors with the major purpose of reducing the number of the electrode pads. Further purposes included lowering the forward voltage of the light-emitting elements, lowering the power consumption, reducing the size of the LED array, and thereby reducing its manufacturing cost.
As shown in FIG. 19, an LED array of the multilayer-wiring type comprises p-wires 32 electrically connected to the LEDs (light-emitting elements) 20, and common wires 60 that interconnect the p-wires 32 so that they are all electrically connected to one or another of the p-electrode pads 33. The p-wires 32 and common wires 60 are disposed in separate wiring layers separated by an inter-layer dielectric film 50. The p-wires 32 and common wires 60 are perpendicular to each other, and the semiconductor layer 12 including the LEDs 20 is divided into a plurality of mutually isolated blocks 10.
An LED array of this multilayer-wiring type can be manufactured by, for example, the following method. First, an n-type semiconductor layer 12 is formed on a high-resistance substrate 11. Next, isolation trenches 13 are formed, dividing the semiconductor layer 12 into M electrically isolated blocks 10. Then a p-type impurity is selectively diffused through a diffusion mask 21 to form N p-type semiconductor regions in each block. Each of these N p-type semiconductor regions becomes an LED. (M and N are integers greater than unity.)
Next, p-electrodes 31, p-wires 32, and p-electrode pads 33 are formed. In each block 10, the p-electrodes 31 and p-wires 32 are in one-to-one correspondence to the N LEDs 20, the p-electrodes 31 forming the ends of the p-wires 32 that contact the LEDs 20. Each block 10 has one p-electrode pad 33, connected to one of the p-wires 32 in the block. After an inter-layer dielectric film (not shown) has been deposited, and this film and the diffusion mask 21 have been patterned, one n-electrode 41 is formed in each block 10, making electrical contact with the n-type semiconductor layer 12. Then one n-wire 42 and one n-electrode pad 43 are formed in each block 10, the n-wire 42 connecting the n-electrode pad 43 to the n-electrode 41. The p-electrode pads 33 and n-electrode pads 43 are used for wire bonding.
Next, an inter-layer dielectric film 50 is deposited and patterned to form openings 51 in which the p-wires 32 are partly exposed. N common wires 60 per cell are then formed, extending across all blocks in the cell and connected to the p-wires 32 through the openings 51. Each common wire 60 is connected to one p-wire 32 of each block 10.
In an LED array with this multilayer wiring structure, the n-electrodes 41 are formed near to LEDs 20 in order to diminish the voltage drop caused by the resistance component of the high-resistance substrate 11. This enables the forward voltage supplied to the LEDs to be reduced, so that power consumption is reduced. In addition, placing the p-electrode pads 33 and n-electrode pads 43 on the same side of the cell reduces the cell size, thereby reducing the manufacturing cost.
In a conventional multilayer-wiring LED array, however, the p-electrode pads 33 and n-electrode pads 43 are disposed alternately. As a result, in the right half of the cell in FIG. 20, the p-wires 32 connected to the p-electrode pads 33 cross the n-wires 42 connected to the n-electrode pads 43. These crossings are potential sites of wiring defects. Another problem is that the crossed wires cannot be formed by patterning the same conductive film, and cannot be formed in the same manufacturing process step. A further problem is that an extra dielectric film is required to insulate the crossed p-wires and n-wire from each other; that is, the p-wires and n-wires must be disposed in separate wiring layers.
An object of the present invention is to reduce the occurrence of wiring defects in an array of light-emitting elements having multilayer wiring.
Another object of the invention is to enable all electrode pads and their connected wiring to be made from the same material in the same step in the manufacturing process.
Another object is to reduce the number of wiring layers in an array of light-emitting elements having multilayer wiring.
The invented array of light-emitting elements is divided into electrically isolated blocks linked by common wiring. Each block includes a row of light-emitting elements, an electrode disposed adjacent and parallel to the row of light-emitting elements, a first pad for attachment of a bonding wire, a first wire electrically connecting the first pad to one of the light-emitting elements in the block, a second pad for attachment of another bonding wire, and a second wire electrically connecting the second pad to the above-mentioned electrode, preferably at the middle of the electrode. The rows of light-emitting elements in the blocks are mutually aligned to form a single row of light-emitting elements.
The relative positions of the first and second pads and wires vary among the blocks. Specifically, the blocks are divided into a first group in which the first pad and first wire are disposed to one side of the second pad and second wire, and a second group in which the first pad and first wire are disposed to the other side of the second pad and second wire. This arrangement enables the first wires to connect the first pads to light-emitting elements in different positions in different blocks without crossing the second wires.
The lack of such crossings reduces the opportunity for wiring defects to occur, and enables the first pad, first wire, second pad, and second wire in each block to be disposed in a single wiring layer. The first pad, first wire, second pad, and second wire can also be made of the same material, in the same manufacturing process step.
The array may be divided into cells, each comprising a plurality of blocks with varying pad and wire arrangements as described above. In this case the common wiring includes common wires interconnecting blocks in the same cell; blocks in different cells are not interconnected. The first wire in each block in the cell is electrically connected to one of the common wires. Each block also includes further wires, formed in the same layer as the first wire, connected to other common wires and to other light-emitting elements in the block.